* Remove dead code from ExprTypeConvert.cpp

* Remove dead code from ExprTypeConvert.cpp
   - I->use_empty() can never be true because of the IHolder's
* Fix bug: test/Regression/Transforms/LevelRaise/2002-07-16-SourceAndDestCrash.ll
   - Add a new NewCasts member to VMC to keep track of casts that have been
     created and to ensure there is always a reference to the cast.
   - Extend ValueHandle a bit so it can be used in an STL container
   - Make sure we destroy the ValueMapCache before verifying the function
     in LevelRaise.cpp

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2936 91177308-0d34-0410-b5e6-96231b3b80d8